This invention is related to excitation leads for connection to a device such as a superconducting magnet. More particularly, the present invention is related to an excitation lead system which provides thermal isolation during normal magnet operation but yet provides a relatively warm contact surface for external connection of electrical leads which are employed when it is necessary to change the magnet field levels or to make field corrections.
In a superconducting device such as a coil of superconducting material which forms part of a magnet assembly, it is necessary to maintain the superconducting material at sufficiently low temperatures. For the present at least, materials which exhibit superconducting properties at room temperature are not known to be available. In order to maintain the superconducting material at the proper cryogenic temperatures, a housing known as a cryostat is employed to provide the desired amount of thermal insulation between the cryogenically cooled superconducting material and ambient conditions. The cryostat employed to achieve this thermal isolation typically includes an inner vessel defined by a set of internal walls and an external vessel defined by a set of external walls, the volume between the two vessels being maintained under vacuum. The interior volume of the cryostat is filled to the desired level with a coolant medium, such as liquid helium, so as to maintain the electrical device contained within the internal cryostat volume at a temperature suitable to maintain the coils in the superconducting state.
An application for superconducting magnets is nuclear magnetic resonance. One of the objects of nuclear magnetic resonance is to provide images of internal body organs without the necessity of exposing the patient to ionizing radiation. Another object is to perform spectroscopy, in vivo. In short, NMR appears to be able to provide physicians and medical technicians with valuable diagnostic information in a manner which is totally non-invasive. In order to increase the resolution of the resultant NMR signals and in order to compensate for the inherently weak nature of the resultant NMR signals, it is highly desirable to place the patient or object being studied in a highly uniform magnetic field which is also a high strength magnetic field. In particular, it has been found that it is generally desirable that this field have a magnetic field strength of between about 0.1 and about 2.0 tesla, or more. There are in general two means for providing such a strong magnetic field. One may employ conventional resistive type magnets in which a large amount of electrical power is consumed because of the high current levels required and the finite or non-zero resistance of the wire. Alternatively, and in keeping with the objects of the present invention, it is also possible to employ a superconducting magnet. In such a device, a coil of superconducting material is maintained inside a cryostat filled with a coolant, such as liquid helium. The choice of superconducting material and coolant employed must of course be compatible so that the material is in fact maintained in a superconducting state when maintained at the temperature of the liquid coolant. Because of the nature of superconducting materials, it is not only possible to create large levels of current flow within these materials, but it is also possible and usually desirable to operate the superconducting magnet or device in what is called the "persistent mode". In this mode, a current is injected into the superconducting coil or the material through which it is caused to flow. The coil terminals are shorted, the driving excitation is then removed and the superconducting nature of the material permits the current that has been established to flow indefinitely. Accordingly, this is described as the persistent mode of operation.
However, it sometimes becomes necessary to change the level of the magnetic field produced by the circulating current. It is also sometimes necessary to change the current in the field in the coil to make corrections in the magnetic field distribution, particularly when the magnet or device has been operating over an extended period of time. When it is necessary to change the level of current circulating within the superconducting material it becomes necessary to connect the coil or device to external leads. It is to these leads that the present invention is directed.
While it is possible to operate superconducting magnets in a fashion in which the external leads are always in contact with an external current source, this is nonetheless undesirable because of thermal losses which can result. In particular, if the coil excitation is maintained by current flowing through the leads, then current flowing in the leads, which are not maintained in a superconducting state, produces a resistive (or I.sup.2 R) heating in these leads. This electrical heating of the external portion of the lead that bridges the interface region between the liquid coolant and ambient temperature can easily result in heating and boiling off of the liquid coolant. If the coils are shorted so that current is not flowing through the leads, but the leads are left in place so as to experience the temperature range between ambient temperature and the cryogenic coolant for the coil, conduction of heat from ambient through the lead can also cause boiling of the liquid coolant. Conventionally, it is found that this heating does in fact cause boiling of the liquid helium coolant thereby producing loss of this expensive coolant. Accordingly, for these reasons it is desirable to construct excitation leads which are repeatedly removable so that this form of coolant heating occurs only during those times which the leads must be used to change the magnetic field. Furthermore, it is seen that a form of connection is desired in which many make and break contact cycles can be made.
For this kind of persistent mode of superconducting magnet operation, the present practice employs an electrical contact interface or joint which is maintained at the liquid coolant temperature. Since heat generated by electrical power loss in contact resistance passes into the liquid coolant medium, it is imperative that the electrical resistance of this joint be extremely low. In testing such joints, it has been found that contact surfaces coated with indium may be employed to attain such low joint resistance. However, indium exhibits a tendency to wear following repeated make and break contact cycles. This wear is caused by the mating part, which is deliberately designed to be harder than the lead and particularly sharp in order to ensure that a low resistance joint can be made in spite of possible water or solid-air frost formation on the cold mating surface. Furthermore, the mating surface of the lead that is maintained at the liquid helium temperature may not be repairable unless the entire cryostat is drained and warmed up to ambient temperature. This is a significant process because it is both time consuming and expensive. It would be particularly undesirable to have to go through this process for superconducting magnets employed in NMR. Accordingly, it would be desirable to be able to employ an excitation lead system for a superconducting magnet operating in the persistent mode which overcomes these operational difficulties.